The present application relates to aerofoils and methods of making aerofoils, particularly aerofoils utilised in gas turbine engines for aerospace applications.
Operation of gas turbine engines is well known and incorporates a need for compressor and turbine blades. Fan blades in smaller engines have traditionally been made from solid titanium as the aerofoil size makes it inefficient or impractical to incorporate a honeycomb or line core internal structure as for larger fan blades. It will be understood that fan blades must deliver a required performance, service life and have sufficient bird-strike worthiness at a reasonable weight.
Solid titanium aerofoil surfaces are defined by the aerodynamic form of the blade, which can lead to a structure that has areas of more than sufficient strength to meet its requirements. In such circumstances, utilisation of solid titanium is both expensive and inefficient with respect to weight conservation. Lighter materials are known, which generally incorporate aluminium or magnesium, but are also weaker and therefore would not be considered acceptable in higher stress regions of an aerofoil.
FIG. 1 provides a schematic side view of a prior approach to providing a solid fan blade which is not constructed from solid titanium. Thus, the fan blade 1 retains titanium 2 on the leading edge 2a for bird strike protection, and at the root 2b and at the lower aerofoil 2c in order to carry the rotating weight of the blade 1. In such circumstances, an upper trailing edge 3 is formed from aluminium. Furthermore, magnesium 4 may also be included in the blade 1. The effective reduction in loading also allows substitution of lighter materials such as aluminium, or even lighter materials such as magnesium, further down the aerofoil and so additionally reduces the thickness of the titanium sections for even greater weight savings. It will be understood that joints 5, 6 between the titanium, aluminium and magnesium sections may be scarfed to increase the bonding area and reduce any effect of sudden changes in stiffness.
Solid titanium blades are functionally disadvantaged purely by their own mass. Excess mass on the aerofoil requires extra material on the blade root, the fan disc and other rotating components to ensure sufficient strength for predictable stresses, and additional thickness on the containment case to contain the blade during a blade-off event. Thus, removing mass from the aerofoil would allow much more mass to be removed from other components, leading to a lighter engine and a greater potential payload and other improvements for an aircraft.
Generally, it is not possible to reduce the mass of a solid titanium aerofoil without life reduction as the lower aerofoil carries the parasitic mass of the tip. Smaller blades are more difficult to manufacture with a hollow titanium internal structure, which makes them cost uncompetitive. Additionally, it will be appreciated that simply substituting a proportion of a titanium aerofoil with aluminium would provide weight savings, but these are limited by the differences in properties between aluminium and titanium. Aluminium is less dense and less stiff than the titanium it replaces, altering the vibrational characteristics and the behaviour of the structure during a bird strike. At joints between the differing metals in particular there is a weakness, as the changing stiffness draws load as the aerofoil is deformed during a bird strike.
A change in stiffness and mass may be optimised for vibration and bird strike to obtain full benefits from substituting some of the titanium with lighter metals. This approach is likely to lead to a re-distribution of material over a far greater proportion of the aerofoil than the upper rearward corner, as pictured in FIG. 1.